h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Military applications of the ramjet - it might be nice to mention (maybe in a footnote) that the (now retired) Lockheed SR-71 Blackbird spy plane used a combination turbojet/ramjet. The engine functioned as a normal jet to get the aircraft up to ramjet speeds (Mach something or other, maybe 2). After that the airflow through the engine was diverted around the turbine section and the engine became a sort of annular ramjet, if you like. This gave a top sustained speed of a bit over Mach 3.
I assume they use something like this in the SR-71 replacement, assuming there is one of course...

Excellent piece of technical detail, thanks very much for that. If and when the entry is updated, I think that should definitely go in. (Although I wrote the first draft of this entry, the other credited authors supplied almost all of the good stuff, so I've learned a lot from writing it, and I just learned some more - cheers! ).

As for the SR-71 replacement - Shhhhhhh!

H.

Why do ramjets need such a high fuel flow at low speeds to produce so little thrust? I've read a ramjet can kick in as slow as 200mph, but they're worse than rockets efficiency wise until about 400 mph. Is that sfc for any thrust or just maximum thrust? Is it the back thrusting tendency that makes the combustion work against itself? Where is all that energy going? Fuel burned at a certain rate generates a certain amount of horsepower, whether the air is compressed or not. A certain amt. of horsepower is capable of a certain amount of thrust when acting on a certain amount of air. I think 4.13# thrust per horsepower.

I can see how high speed & high compression may be necessary to get high thrust & acceleration. But what about efficiency at lower thrust levels that are sufficient for flight at those speeds? Is it possible to design a nonmechanical, turbofan-grade efficient propulsion system in the 200 mph range or slower? All things being equal, it takes less horsepower(less fuel) to produce the same thrust at lower speeds, especially if there's a lot of propellant mass to work with.

I think I see why replies are taking a while. I'm the 1st one to post ANYTHING here since 2001!